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March 3, 1964 D. s.-PoKRAs ETAL ELECTROLESS GOLD PLATING SOLUTION 2 Sheets-Sheet l Filed DeC. 15, 1960 ON Qmm Own 0mm 00m ONN O wN O N O Om- ON Om .Ow Om OO A Thomas L. Sullens, Robert F. Walton 'BY fm/ QNN March 3, 1964 D. s. PoKRAs ETAL ELEOTROLESS GOLD PLATINO SOLUTION Filed Dec.

2 Sheets-Sheet 2 INVENIORS Donald S. Pokros,

Thomas L. Sullens', Robert F. Wolion y United States Patent O 3,i23,1t84 ELECTROLESS GOLD PLATIN@ SOLUTlON Donald S. Pelotas, Thomas L. Sullens, and Robert F. Walton, Dallas, Tex., assignors to Texas Instruments Incorporated, Dallas, Tex., a corporation ot elaware Filed Dec. 15, 1960, Ser. No. 76,036 4 Claims. (Cl. 10G-d) This invention relates to an improvement in the deposition of gold on a substrate. More specifically the invention relates to an improved electroless gold plating solution and to the method of preparing the saine. I

The three types of gold plating processes now available are electrolytic gold plating, the immersion processor gold plating and the electroless process of gold plating.

The electrolytic process of gold plating is carried out when the metal being plated transfers an electron to the gold anion, thus converting the gold anion to metallic gold at the metal surface and concurrently depositing the gold thereon. Any thickness of gold may be plated onto a metal surface by this process. One disadvantage of the electrolytic process is that the gold plating is not uniform over the surface of the metal body if the surface is irregular. This means that sharp points of the metal body will be plated more heavily than the rest of the body.

The immersion process of gold plating is carried on by a chemical process. Initially, a chemical in the gold plating solution prepares the surface of the metal 1n such a way that a gold layer can be deposited on the metal. However, after the irst layer of gold is deposited on the metal, the process stops. This is because the chemical reaction can no longer take place since the gold has covered the base metal surface.

The electroless gold plating process is characterized by the fact that the amount of gold deposited on the metal is proportional to the time that the metal remains immersed in the plating solution. A continuous plating action in an electroless solution is made possible by the fact that a constituent thereof continuously activates the gold surface that has already been plated onto the base metal.

It is to the last-named type of plating process that this invention relates. ln the past, various solutions have been proposed by which to effect electroless plating of gold. The various solutions proposed have been subject to one or more disadvantages, principal among which has been the inability to produce a plating or uniform thickness and/or substantial thickness. Another drawback to electroless gold plating solutions heretofore known is that they produce coarse grain platings. It is to overcome these disadvantages that the present invention was conceived. By means of the present invention, a method of preparing an electroless gold plating solution and a novel gold plating solution are provided whereby it is possible to conduct an electroless plating process to produce gold coatings having superior properties. By using the cornposition of the present invention experimental data shows that almost any uniform thickness of gold can be plated by the process and that the plating will be characterized by an exceptionally line grain.

It is therefore an object of the present invention to provide an electroless gold plating solution with a constant deposition rate.

It is another object of the present invention to provide a method for the electroless deposition of gold onto a substrate of a less noble metal to any useful desired thickness.

It is still a further object of the present invention to provide a method of preparing an electrolessly gold plating solution for depositing gold onto a less noble metal substrate, whereby the deposited gold layer has a uniice form thickness, a line grain structure and a much greater thickness than heretofore attainable.

lt is a feature of the present invention to provide an electroless gold plating solution containing an amino acid or water soluble salt thereof.

It is still another feature of the present invention to provide an electroless gold plating solution containing hydroxy dicarboxylic or hydroxy tricarboxylic acids and an amino acid or a water soluble salt thereof.

It is still a further feature of the present invention to provide an electroless gold plating solution wherein a tungsten compound is added as a grain reiiner.

Other and further objects and features of the present invention will become evident from the following detailed description of the preferred mode for carrying out the invention.

The constituents of the gold plating solution are a water soluble gold salt, an acid from the general groupings of hydroxy dicarboxylic or hydroxy tricarboxylic acids and an amino acid or a water soluble salt of an amino acid. In general, the above three components when properly mixed result in an acidiiied gold solution containing the novel ingredient, that is, an amino acid or a salt thereof, that promotes or stimulates the electroless process. Various theories have been proposed as to the action due to the addition of the salt of an amino acid, one being that it furnishes the electrons to reduce the gold at the metal eing plated, thereby allowing continued deposition of gold to any desired thickness. For additional features two other important components may be added. The plating solution will more readily retain its desirable plating characteristics if a butler solution is used to maintain the pH of the soluton within a given range. The butler prevents the pH of the solution from becoming relatively too basic. A solution of pH exceeding a maximum value results in the rate of deposition of the gold being retarded and in some cases ceasing altogether. Last, the addition of a grain reiiner is, in many instances, desirable to give a closely-packed ne grain deposit.

The preceding indicates the general class of constituents necessary for an electroless gold plating solution. Preferred compounds, salts and acids of the general classes ot constituents will indicate desired plating solutions. For example, potassium gold-cyanide [KAMCNM] can be used to furnish the gold content of the solution. Theoretically, potassium-gold-cyanide contains 68.3 percent gold by Weight although impurities may reduce the percent weight of gold to a lower value. Gold cyanide (AuCN), sodium-gold-cyanide [NaAu(CN)2] and gold compounds with the addition of potassium cyanides or gold cyanides can be used to furnish the gold content of the plating soluion.

To obtain the desired plating action the plating solution must be at least acidic and buffered so that the solution does not exceed a given pH, thereby becoming too basic. A mild organic acid such as citric acid or tartaric acid will render the solution acidic as desired. Other organic acids can be used although the two acids mentioned give a preferred quality. For example an acid from the general groupings of hydroxy dicarboxylic or hydroxy tricarboxylic acids will work.

In order to obtain a line grain deposit of gold a tungsten compound is added as a grain rener. It has been found that tungstic acid works very Well as a grain retiner. Other compounds such as orthotungstic acid, tungsten oxide or tungsten dioxide will work. Such compounds as sodium tungstate and sodium paratungstate will suflice. Also, potassium tungstate, potassium paratungstate or potassium metatungstate can be included.

For a proper plating action the solution must be buffered to a ygiven pH value. It has been found that the addition of phthalic acid monopotassium salt-sodium hydroxide suresnes successfully buffers the solution to a pH betvveen 5.() and 6.9. Other buering agents will work such as acetic acid-sodium acetate, builering at a pH of 4.6, secondary sodium citrate, bulering at pH of 5.0, potassium acid phosphate-disodium phosphate, builering at a pH of 6.8 or potassium acid phosphate-sodium hydroxide, bull-ering at a pl-l ot 6.8. The plating solution best periorms at a pH value between 5.0 and 6:0. The preferred battering agent is plithalic acid monopotassium salt-sodium hydroxide.

Finally, an amino acid or a Water soluble salt of an amino acid is necessary to give the electroless plating charcteristic instead of an immersion characteristic. The preferred agent used is a salt of an amino acid, tl. salt being `l`LN-diethylglycine sodium salt, although compounds such as N,N-dipl1enylglycine sodium salt, NJT- dimethylglycine sodium salt and N-phenf'lgl fcine pote."- sium salt will Work.

The plating solution is composed of four parts, and these parts are mixed according to the procedure set out, as will be shown later. The general procedure for mixing will be given although various percentages will be indicated subsequent to the mixing procedure. The approximate amounts of deionized Water are given in the `following procedure for a volume of one liter of solution. Although the percentages by weight of the constituents may be changed, the same amounts of water are usually used.

Solution A T'ne water soluble gold salt (preferably potassium gold cyanide) is dissolved in about 50G cc. of deionized Water. Separately, an acid from one of the general groupings of hydroxy dicarboxylic acid or hydroxytricarboxylic acid is dissolved in about 250 cc. of deionized Water, the acid preferably being tartaric acid or citric acid. The tartaric acid or citric acid solution is added to the potassium gold cyanide solution and mixed thoroughly. The acidilied potassium gold cyanide is heated to about 140 F. and subsequently cooled to room temperature. Then the solution volume is increased to about SGO cc. with deionized Water.

Solution .l-An alkali base (preferably sodium hydroxide) is dissolved in about l() cc. of deioni" d Water and cooled to room temperature. Tbe cationic compound (preferably tungstic acid) is dissolved in the alkali base solution. To this solution is added the saine carboxylic acid as added in solution A, in this case tarta-.ric or citric acid. Deionized Water is added until the solution volume is about 150' cc.

Solution G An allzali base is dissolved in about cc. of deionized water-sodium hydroxide if also used in solution B. The other constituent of the buffering agent (preferably phthalic acid monopotassium salt) is dissolved in the above al1/.ali base solution.

Soll/.tion D.-The amino acid or water soluble salt of an amino acid (preferably N,N-diethylglycine sodium salt) is dissolved in about 10 cc. of deionized Water.

To combine these solutions it has been found that the following procedure gives lbest results. Solutions A and B are mixed and the pli adjusted with the same alltali base or acid previously used to a value of between 4.6 and 6.8. That is, if the pH is adjusted to a more acidic value, tartaric acid or citric acid is used, the one used being the same as previously used in solutions A and B. lf the pH is adjusted to a more basic value, sodium hydroxide is used if it were used previously. The exact amount or these additives cannot be specified but must be added in a quantity to adjust the pH properly.

Solution C is added to the adjusted solution of A and B. Then solution D is added to the combination oi solutions A, B, and C.

During the mixing process the sodium hydroxide serves three purposes. First, tbe grain renner commonly used, Efor instance, tungstic acid, is not easily dissolved in an acid, but is more easily dissolved in a base such as sodium hydroxide, Secondly, the sodium hydroxide is one nized l component of the butler solution. Thirdly, the pH oi' the plating solution can be adjusted by the addition of sodium hydroxide.

order to show preferred embodiments of the present invention specific percentages of tbe various constituents will be given although variations of these percentages can be used to prepare solutions that will Work to varying degrees of optimization Without departing troni the scope of the invention. For example, the gold cyanide may be from 7 to 28 grams per liter. Tbe citric acid or tartaric acid may be from 5 to 6()` grams per liter. The tungstic acid may be from 5 to 90 grams per liter. The ratio of potassium acid phthalic to sodiu i hydroxide may be 5.10 to l by weight in solution. The N,Ndi ethylglycine sodium salt may be from one-half to l5 per liter.

it has been found experimentally that only slight variations in results are obtained when comparing a solution containing citric acid and a solution containing tartaric acid. To illustrate, the optimum chemical compositions of two solutions, one containing tartaric acid and the other containing citric acid, are identical. Experimentaliy determined, the optimum chemical composition of electro-less gold plating solutions are as follows:

Citric Acid Tartaric Acid Concentration of- Solution in Solution in grams/liter grams/liter Potassium Gold Cyanide..A 28 28 Citric Acid 60 None Tartaric AoitL None G0 45 Q5 16 16 N, N-Dietliylglycine Sodium Salt 3. 3. 75 Phthalic Acid Monopotassium Salt. 25 25 The pH of the above solutionts) is adjusted with citric acid or tartaric acid, or sodium hydroxide to have a pH value of 5.0 to 6.0.

To illustrate the results of using these solutions the following data show the amounts of g ld deposited from a citric acid and a tartaric acid electroless gold solution when used to plate both ltovar metal and nickel metal.

TEST RESULTS FROM CITRIC ACID BASE ELECTROLESS GOLD SOLUTION TEST RESULTS FROM TARTARIC ACID BASE ELECTROLESS GOLD SOLUTION 5 O. 955 Kovar 1() 1. 304 Kovar 15 0113 30 150 G0 3. 954

l2' 11.998 240 2l. O15 360 l?. 131

5 0. 92S 1l) 1. 243 15 1.503 30 3, 115 GO 5.169

FIGURE 1 shows a graph of Weight of gold deposited in mg./ sq. in. versus immersion time in minutes for the citric acid base solution when plated on Kovar and nickel. FGURE 2 sho-ws a similar graph for the tartaric acid base solution. Both graphs show curves of a linear nature with evidently no limit as to the amount of gold that can be plated ont-o a metal when immersed for a sufficient length of time. These results (characteristic of an electroless solution) are distinct when contrasted with the results obtained by using an immersion process.

Other examples of composition percentages that Will work but do not deposit as much gold per unit of time are as follows:

The pH of the above solution(s) is adjusted with citric acid or tartaric acid, or sodium `hydroxide -to have a pH value of 5.0 to 6.0.

In the above examples, the 'other speciiic compounds. salts and acids of the general classes of constituents listed supra could have been used. Also the percentage of each component used can be varied Within limits. The procedure as set forth `for mixing the components to form the solutions will remain substantially the same, however.

It is to be understood that, although the invention has been described with specific reference to particular embodiments thereof, it is not to -be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as deiined by the appended claims.

What is claimed is:

1. An electroless gold plating solution comp-rising an admixture 7 to 28 grams per liter of potassium gold cyanide, 5 to 60 grams per liter of an organic acid seleoted Ifrom the group consisting of citric acid and tartaric acid, 5 to 90 grams per liter of tungstic acid, and a ratio of 5.10:l by weight in solution of potassium acid phthalate to sodium hydroxide, and l/z to 15 grams per liter of N,Ndiethylg1ycine sodium salt.

2. An electroless gold plating solution comprising an admixture of 28 grams per 'liter of potassium gold cyanide, 60 grams per liter of an organic acid selected from the group consisting of citric acid `and tartaric acid, grams per liter of tungstic acid, 16 grams per liter of sodium hydroxide, 25 grams per liter of phthalic acid monopotassium salt and 3.75 grains per liter of N,Ndiethyl glycine sodium salt.

`3. An electroless gold plating solution comprising an admixture of 7 grams per liter of potassium gold cyanide, grams per liter of tartaric acid, 45 grams per liter of tungstic acid, 16 grams per liter of sodium hydroxide, 25 grams per liter of phthalic acid monopotassium salt and 3.75 grams per liter of N,Ndiethylglycine sodium salt.

4.y An electroless gold plating solution comprising an admixture of 14 grams per liter of potassium gold cyanide, l5 grams per liter of citric acid, 16 grams per liter of sodium hydroxide, 25 grams per liter of phthalic acid monopotassium salt and 3.75 grams per liter of N,N diethylglycine sodium salt.

References Cited in the le of this patent UNITED STATES PATENTS 2,819,187 Gutzeit etal lan. 7, 1958 2,836,515 McNally et al May 27, 1958 2,969,295 Crishal Ian. 24, 1961 2,976,181 `Broo'kshire Mar, 21, 1961 

